A number of companies manufacture a variety of voltmeters, including high-voltage meters. The high-voltage meters may be of the type generally called hot-stick voltmeters or hot-stick voltmeters/phasers. They generally incorporate the same basic elements, i.e., two hot-sticks with high-voltage resistors encapsulated therewithin, a connecting cord between the two hot-sticks and a voltage meter display. Although meters of this type generally have the capability and resolution of measuring voltages as low as 10 volts and as high as 100 kV, they are most commonly used to measure voltages in the range of 1 kV to 80 kV and are used most commonly on electric power distribution systems of the type operated by electric utility companies.
The high-voltage resistors are selected for their very high electrical resistance, generally several megohms or more, and their ability to withstand the application of high voltage with voltage ratings up to 50 kV per resistor. These resistors are encapsulated within supporting members, generally cylindrical, and these members are constructed of a material selected for both its ability to withstand the direct application of high voltage and mechanical ruggedness to protect the typically more fragile high-voltage resistors inside. The encapsulating material between the resistors and the supporting members serves to provide additional mechanical protection for the resistors, to provide a thermal path for heat generated by the resistors to the outside world and to ensure that the space surrounding the resistors remains free of any contaminant, such as water, which would reduce their capacity to withstand high voltage. The encapsulating material is generally selected for properties such as ability to withstand high voltage, a wide range of operating temperatures and good thermal conductivity. The tubular supporting members, with the encapsulation for the high-voltage resistors therewithin, are often called “hot-sticks,” but they are different from the accessory hot-sticks described below.
The connecting cord between the two encapsulated resistor members carries the current from one resistor to the other and is generally covered with an outer high-voltage insulating material, such as rubber, that can withstand the application of the voltages that the voltmeter/phaser will be used to measure.
Mechanical connections on the ends of the tubular supporting members are used to connect these members to accessory hot-sticks. Hot-sticks are long tubular insulating rods, usually several feet in length, that are used to provide both a handle for the users of the voltmeter/phaser and to provide a safe working distance between the user and the energized high-voltage devices or equipment that the voltmeter/phaser will be contacting. The process of measuring live high voltage circuits includes risks from electrocution and burns and the distance that the hot-sticks provide between energized circuits and the persons performing the testing is one component of the safety procedures used to minimize these risks.
Probes on the ends of the meter hot-sticks are typically simple metal terminals and act as a rugged means of contacting the energized conductors or equipment to be measured and connecting this equipment to the encapsulated high-voltage resistors. These probes generally connect to the ends of the hot-sticks with a simple threaded mechanical and electrical connection and are designed to be easily replaced with other probes of different sizes and shapes for different applications.
In operation, the voltmeter/phaser is used to measure the magnitude of a high voltage by applying the metal probes respectively to two different points in a circuit, such as two conductors on a high-voltage circuit. These two conductors can be two phases of a typical three-phase power distribution circuit or any one phase and ground.
When the voltmeter/phaser contacts both conductors of the high voltage circuit a current flows through the voltmeter/phaser, the magnitude of that current being directly proportional to the applied voltage and determined by the value of the high-voltage resistors encapsulated in the hot-sticks. The value of the resulting current is measured and displayed as an equivalent voltage on a suitable meter display.
The display can be either a traditional electromechanical analog type with a moving needle on a printed scale or it can be an electronic digital display of LED or LCD type. Other types of displays, such as bar graphs, etc., are also possible but not common.
Prior voltmeters/phasors of the type described above have only one voltage meter display. That meter display is generally permanently mounted to one of the two hot-sticks.
Voltmeters/phasers are commonly used by a two person line crew, with each person of the crew holding one of the two sticks. Thus, only one of these two persons can readily see the voltage meter display. Because the display is generally visible to only one of the two crew members, various procedures have been developed for the person holding the stick with the meter to relay to the second person the pertinent voltage information.